Oaslm and a method and apparatus for driving an oaslm

ABSTRACT

A method of controlling the switching of an optically addressable spatial light modulator (OASLM), to a first surface of which a write light signal is applied and to a second surface of which a read light signal is applied. The method comprises applying a bipolar switching waveform to control electrodes of the (OASLM) during each write cycle such that the leading pulse of the waveform applies a voltage across the (OASLM) which is in the photosensitive direction and the trailing pulse applies a voltage which is not in the photosensitive direction.

The present invention relates to optically addressable spatial lightmodulators and to a method and apparatus for driving opticallyaddressable spatial light modulators. The present invention also relatesto holographic displays comprising optically addressable spatial lightmodulators.

It is well known that a three-dimensional image may be presented byforming an interference pattern or hologram on a planar surface. Thethree-dimensional image is visible when the hologram is appropriatelyilluminated. Recently, interest has grown in so-called computergenerated holograms (CGHs) which offer the possibility of displayinghigh quality images, which need not be based upon real objects, withappropriate depth cues and without the need for viewing goggles.Interest is perhaps most intense in the medical and design fields wherethe need for realistic visualisation techniques is great. Typically, acomputer generated hologram involves the generation of a matrix of datavalues (each data value corresponding to a light transmission level)which simulates the hologram which might otherwise be formed on a realplanar surface. An important component of the CGH system is an opticallyaddressed spatial light modulator (OASLM). This device comprises aphotosensor layer, typically silicon (Si), and a liquid crystal (LC)layer with electrodes by which a voltage is applied across both layers.When a pattern of light of appropriate wavelength is incident onto thephotosensor its resistance falls and allows most of the applied voltageto drop across the LC which is then switched in those areas which areilluminated A light beam incident on the front of the LC layer is thenmodulated by the LC and reflected from the Si (or from an incorporatedmirror) in such a way that an image is presented to a viewer.

A holographic display system employing CGHs is described in GB2330471Aand is further illustrated in FIG. 1. The illustrated approach is knownas Active Tiling™, and involves the use of a relatively smallElectrically Addressable Spatial Light Modulator (EASLM) 1 incombination with a relatively large Optically Addressable Spatial LightModulator (OASLM) 2. The holographic matrix (CGH) is subdivided into aset of sub-holograms, with the data for each sub-hologram being passedin turn to the EASLM 1. The EASLM 1 is illuminated from one side withincoherent light 3. The OASLM 2 comprises a sheet of liquid crystal (inone example the liquid crystal is a bistable ferroelectric liquidcrystal) which is switched from a first to a second state by a suitablevoltage in the presence of incident light In one method, Guide optics 4,which may include a shutter, disposed between the EASIM 1 and the OASLM2, cause the output of the BASLM 1 (i.e. light transmitted through theEASIM 1) to be stepped across the surface of the OASLM 2. The bistablenature of the OASLM liquid crystal means that the portion or “tile” 5 ofthe OASLM 2 onto which a sub-holographic image is projected andappropriate switching voltage applied, remembers that image until suchtime as the OASLM is reset by the application of an electrical voltage.It will be appreciated that, when all of the component images have beenwritten to the OASLM, the OASLM will have “stored” in it a replica ofthe complete holographic matrix. The image may be replayed by a coherentbeam incident 6 on the LC. When the next holographic image is ready tobe transferred to the OASLM a reset voltage is applied to remove theexisting image from the OALSM and prepare it for loading the next image.The holographic display also typically comprises a large output lens,although this is not shown in FIG. 1.

The structure of a typical OASLM is illustrated in FIG. 2 and asimplified equivalent circuit is shown in FIG. 3. In this circuit thevoltage is applied to the Si side of the device and the LC side isearthed. This convention will be followed throughout the followingdiscussion. From left to right in FIG. 2, the layers are as follows; afirst glass layer 1, an indium tin oxide layer 2 which forms a firsttransparent electrode, a silicon photosensor layer 3, a light blockinglayer 4, a mirror 5, a first alignment layer 6 which may be formed bybrushing a polyimide layer, a liquid crystal (LC) layer 7, a secondalignment layer 8, a second indium tin oxide layer 9, and a second glasslayer 10. A voltage source 11 is coupled to the two indium tin oxidelayers 2, 9 in order to control the switching of the OASLM. Thesilicon-indium tin oxide layer junction acts as a diode; when a voltageof a first positive polarity is applied across the device this diode isforward biased and most of the voltage will be dropped across the LClayer 7, whilst when a voltage of a second, negative polarity is appliedacross the device, most of the voltage will be dropped across thesilicon layer 3 unless write light is applied in which case the voltagewill be dropped across the LC layer 7. The bias of the second polarityis referred to as the “photosensitive direction”. When the bias is inthe photosensitive direction and with no illumination, the voltageappearing across the LC layer 7, V_(lc), is given by the capacitivedivision of the total voltage appearing across the OASLM:V _(lc) =C _(Si)/(C _(lc) +C _(Si)),where C_(Si) and C_(lc) are the capacitances of the silicon and LClayers respectively. As charge is generated in the Si layer, so thevoltage across the LC rises.

In the ideal case a Schottky barrier is formed in the OALSM by thesilicon and indium-tin-oxide (ITO) transparent electrode. This givesbehaviour some way between that of a photodiode and a photoconductor. Ifohmic contacts are made then photoconductor behaviour results. The majorproblem with a pure photoconductor is the dark leakage current which isnot sufficiently low to keep the voltage from dropping across the LC ina non-illuminated addressed state. A photodiode requires the depositionof p-doped, intrinsic and n-doped Si and is a complicated process. For aphotodiode under reverse bias, when a photon is absorbed to produce anelectron-hole pair in the Si, the hole and electron are separated anddrift to the contacts. The blocking contacts stop the carriers so thatonce they are collected the response is complete. The photocurrentvaries linearly with the light intensity over a wide range ofintensities because one electron-hole pair is collected for eachabsorbed photon. With the application of a positive applied voltage thephotodiode is forward biased so that all of the voltage should dropacross the LC. The presence of a write light should not affect the stateof the LC significantly, with a positive voltage applied. When anegative applied voltage is applied, the photodiode is reverse biased,blocking the current, so that ideally the voltage across the LC isunchanged. When a write light illuminates the photodiode a photocurrentcharges the LC to a negative voltage and causes switching. This voltageis maintained across the LC until the drive voltage goes positive again.

For more details of the operating theory of spatial light modulatorssee, for example, “Spatial Light Modulator Technology, Materials,Devices and Applications”, edited by U Efron, published by Marcel DekkerInc. 1995 and “Optimisation of ferroelectric liquid crystal opticallyaddressed spatial light modulator performance”, Perennes F & Crossland WA, Opt. Eng. 36 (8) 2294-2301 (August 1997).

A typical voltage signal for controlling an OASLM is illustrated in FIG.4. Prior to each write phase, a blanking pulse is applied to the device.The blanking pulse has a relatively large amplitude and duration and hasa polarity in the opposite direction to the polarity of thephotosensitive direction (i.e. positive). Typically, the whole devicemay be illuminated entirely with write light for the duration of theblanking pulse. The blanking pulse results in the molecules of theliquid crystal layer being oriented in a first direction. Each blankingpulse is followed by a write pulse which has a polarity in thephotosensitive direction, the write pulse may be separated in time fromthe blanking pulse or may follow it immediately. There will usually besome means of DC balancing the blanking pulse and write pulse so thatthere is net zero DC, but this is not shown in the Figure. During awrite pulse, the silicon side of the device is illuminated with thedesired pattern. The result is that the applied voltage is droppedacross the liquid crystal layer in those regions where the device isilluminated, causing the liquid crystal to switch to a second state. Innon-illuminated regions of the device, the liquid crystal does notswitch. The “switched” pattern in the liquid crystal is used to modulatea light beam incident on the liquid crystal side of the device.

Prior art drive waveforms are described in, for example “Optimisation offerroelectric liquid crystal optically addressed spatial light modulatorperformance”, Perennes F & Crossland W A, Opt. Eng. 36 (8) 2294-2301(August 1997). In this example the erase pulse is immediately followedby a write pulse. This appears to be a simple bipolar pulse but it mustbe remembered that in fact it is two adjacent monopolar pulses ofdifferent polarity and different function, which may be separated intime. Applied Optics Vol. 31, No. 32, pp. 6859-6868, 10 Nov. 1992,describes both erase and write pulses which are bipolar. These may beapplied with either polarity, i.e. leading part positive, training partnegative, or vice versa, since the device used has ohmic contacts. Thefunction of the bipolar pulse in this example is to maintain DC balance.

Whilst a write pulse having a polarity in the photosensitive directionwill be most effective at switching illuminated areas of a device, therewill be a tendency for non-illuminated areas to switch since they willreceive a reduced voltage of the same polarity. Careful design of thewrite pulse (shape, amplitude and width) is therefore required in orderto achieve maximum discrimination between switching of illuminated andnon-illuminated regions. In addition, it is desirable to maximise theregion of pulse amplitude-width space in which the device is operated inorder to compensate for variations within the device, e.g. cell spacing,and in operating conditions, e.g. temperature. Furthermore it isdesirable to maximise switching speed (minimise switching pulse width)in order that images may be updated rapidly, e.g. for frame sequentialcolour. The relative thicknesses of Si and LC layers also influenceswitching since this changes their capacitance and thus the proportionof voltage appearing across the LC due to capacitive division ofvoltage. These parameters will affect the switching characteristic ofthe OALSM although not that of the LC. All of these parameters need tobe optimised.

It is an object of the present invention to increase the range of pulseamplitude-widths within which appropriate discrimination betweenswitching of illuminated and non-illuminated regions is achieved whilstmaintaining or improving switching speed.

When the OASLM is used in a holographic display system, particularly anActive Tiling™ system, the resolution of the written image is important.It has been shown (G Moddel, “Ferroelectric liquid crystal spatial lightmodulators”, Spatial Light Modulator Technology: materials, devices andapplications, E Effron ed., pp. 287-360, Marcel Dekker Inc., New York,1995) that the product of image size and viewing volume is proportionalto the number of pixels contained in the computer generated hologram.Pixels of the order of 10 μm in the OASLM are required. It is a furtherobject of the present invention to improve or maintain thediscrimination between switched and non-switched areas of LC as the sizeof the pixel is reduced.

According to a first aspect of the present invention there is provided amethod of controlling the switching of an optically addressable spatiallight modulator (OASLM), to a first surface of which a write lightsignal is applied and to a second surface of which a read light signalis applied, the method comprising:

-   -   applying a bipolar switching waveform to control electrodes of        the OASLM during each write cycle such that the leading pulse of        the waveform applies a voltage across the OASLM which is in the        photosensitive direction and the trailing pulse applies a        voltage which is not in the photosensitive direction.

Preferably, the shape and amplitude of the bipolar pulse are such thatthe trailing pulse causes switching between stable states, whilst theleading pulse causes substantially no switching between stable state.This switching between stable states is known as “latching”.

The switching waveform may have an asymmetric shape. In particular, theleading and trailing pulses may have different amplitudes and/ordurations. More preferably, the duration of the leading pulse is lessthan that of the trailing pulse and/or the amplitude of the leadingpulse is less than that of the trailing pulse.

By appropriately selecting the pulse shapes, the amplitudes of thepulses can be reduced and/or the size of the operating window in thevoltage—time plane of the switching characteristic can be increased. Alower operating voltage has many advantages including simpler andlower-cost drive chips, increased device lifetime and lower powerconsumption, whilst a widening of the operating window eases constraintson device parameters such as uniformity of cell spacing (leading tovariations in electric field), temperature variation within the device,and varying distortions of the applied voltage pulse in different areasof the device due to resistance and capacitance in the device.

Preferably, the pulse width ratio between the leading pulse and thetrailing pulse is at least 1:4, more preferably at least 1:10.Alternatively or in addition the relative amplitudes of the leading andtrailing pulses may be varied to optimise switching discrimination.

In this first aspect of the invention the OASLM used may operate as aphotodiode having a blocking contact and asymmetric current-voltageresponse.

According to a second aspect of the present invention there is provideda method of controlling the switching of an optically addressablespatial light modulator (OASLM), to a first surface of which a writelight signal is applied and to a second surface of which a read lightsignal is applied, the method comprising:

-   -   applying a bipolar switching waveform to control electrodes of        the OASLM during each write cycle, one of the pulses of the        switching waveform causing illuminated areas of the OASLM to        substantially switch from a first to a second state whilst        causing substantially no switching of unilluminated areas, and        the other pulse of the bipolar waveform causing unilluminated        areas of the OASLM to substantially switch from the second to        the first state whilst causing substantially no switching of        illuminated areas.

The amplitudes and widths of the waveform pulses are selected to ensureswitching of illuminated and unilluminated areas of the OASLM todifferent states. More preferably, the pulse amplitudes and widths arechosen to lie within that region of pulse amplitude/width space which issubstantially bounded by:

-   -   a) a line defining between 95% and 100% switching of illuminated        areas to said first pulse of the bipolar waveform; and    -   b) a line defining between 0% and 5% switching of unilluminated        areas to said second pulse of the bipolar waveform or    -   a) a line defining between 95% and 100% switching of        unilluminated areas to said first pulse of the bipolar waveform;        and    -   b) a line defining between 0% and 5% switching of illuminated        areas to said second pulse of the bipolar waveform

In certain embodiments of the present invention, each bipolar switchingwaveform is preceded by a blanking pulse which causes switching of theentire OASLM to either said first or second state. During the blankingpulse said first side of the OASLM may be completely illuminated.However, providing that the switching discrimination of the leading andtrailing pulses of the switching waveform is sufficient, the blankingpulse is not necessary.

In this second aspect of the invention the device may operate as eithera photodiode or a photoconductor.

According to a third aspect of the present invention there is provided amethod of controlling the switching of an optically addressable spatiallight modulator (OASLM), to a first surface of which a write lightsignal is applied and to a second surface of which a read light signalis applied, the method comprising:

-   -   applying an asymmetric bipolar switching waveform to control        electrodes of the OASLM during each write cycle.

According to a fourth aspect of the present invention there is providedan optically addressable spatial light modulator (OASLM) which in use isarranged to have a write light signal applied to a first surface thereofand a read light signal applied to a second surface thereof the OASLMcomprising:

-   -   a ferroelectric liquid crystal which gives the OASLM device a        response time for switching between the first and second states        which depends upon the voltage applied across the OASLM and the        response time having a minimum value at a given voltage.

In this fourth aspect of the invention the device may operate either aphotodiode or a photoconductor. In use, the switching pulse may bemonopolar or bipolar. Where the switching pulse is monopolar, the pulsepreferably causes unilluminated areas to switch and does not switchilluminated areas Where the switching pulse is bipolar, the leading partof the bipolar pulse preferably causes switching in the latchingdirection and the trailing part does not switch in the oppositedirection.

The use of a ferroelectric liquid crystal (FLC) mixture having a minimumresponse time for switching at a particular voltage (a τv_(min)mixture), provides for a large operating region in the voltage-timeplane which can be accessed at voltages higher than that for minimumpulse width. It should be noted that at voltages higher than the voltagefor the minimum pulse width and at a particular pulse width,discrimination between a switching and a non-switching voltage occurswhen the lower voltage switches and the higher voltage does not switchHence in this mode of operation the un-illuminated areas of the OALSMswitch while the illuminated areas do not switch.

The various aspects of the invention described above may be combined toprovide an OASLM system which has low voltage requirements and whichoperates with appropriate discrimination over a range of operatingconditions and over a range of device tolerances with short pulsewidths.

Other aspects of the invention relate to display systems employing theabove methods and are defined in the attached claims.

For a better understanding of the present invention and in order to showhow the same may be carried into effect reference will now be made, byway of example, to the accompanying drawings, in which:

FIG. 1 illustrates schematically an Active Tiling™ holographic displaysystem;

FIG. 2 illustrates schematically an OASLM of the display system of FIG.1;

FIG. 3 illustrates a simplified equivalent circuit of the OASLM of FIG.2;

FIG. 4 illustrates a typical control signal applied to the OASLM of FIG.2;

FIG. 5 illustrates a switching waveform sequence of the prior art forapplication to an OASLM;

FIG. 6 illustrates an alternative switching waveform sequence of theprior art for application to an OASLM;

FIG. 7 illustrates an improved switching waveform sequence forapplication to an OASLM;

FIG. 8 illustrates an alternative improved switching waveform sequencefor application to an OASLM;

FIG. 9 illustrates a further alternative switching waveform sequence forapplication to an OASLM;

FIG. 10 illustrates a further alternative switching waveform sequencefor application to an OASLM;

FIG. 11 illustrates two switching waveform sequences for application toan OASLM;

FIG. 12 shows a diagrammatic representation of side and end views of FLCmolecules in a cell with thin spacing (such as that of FIG. 2) where 38& 42 are cell walls, 50 is the smectic layer and D is the LC director;

FIGS. 13A and 13B show a diagrammatic view of DC switched states andswitching process for a ferroelectric liquid crystal;

FIG. 13C shows a schematic graph illustrating the dependence offerroelectric torque acting upon the director as a function of thedirector position shown in FIG. 13B;

FIG. 14 illustrates the effect of varying the width of the leading partof a bipolar pulse on the switching response using a computer model inwhich the leading pulse is in the photosensitive direction;

FIG. 15 illustrates the effect of varying the amplitude of the leadingpart of a bipolar pulse on the switching response with three differentratios of the width of the leading part to the trailing part using acomputer model in which the leading pulse is in the photosensitivedirection;

FIG. 16 illustrates a switching waveform sequence comprising a pair ofblanking pulses, 14 & 15, each followed by a bipolar switchingwaveforms, 12 & 13, for application to an OASLM and the resultingoptical response;

FIG. 17 shows the switching response to the second of the switchingwaveforms of FIG. 16, i.e. having a positive trailing pulse, not in thephotosensitive direction, for the cases where the cell is unilluminated.

FIG. 18 shows the switching response to the second of the switchingwaveforms of FIG. 16, i.e. having a positive trailing pulse, not in thephotosensitive direction, for the cases where the cell is illuminated;

FIG. 19 is a combination of the traces of FIGS. 17 and 18 for the caseof Si biased positive and on which is identified a region of goodswitching discrimination between illuminated and unilluminatedconditions;

FIG. 20 shows traces corresponding to the first of the switchingwaveforms in FIG. 16, i.e. having a negative trailing pulse, thephotosensitive direction, and on which is identified a region of goodswitching discrimination between illuminated and unilluminatedconditions;

FIG. 21 illustrates a second switching waveform sequence comprising apair of bipolar switching waveforms for application to an OASLM and theresulting optical response;

FIG. 22 shows the switching response to the second of the switchingwaveforms of FIG. 21, i.e. having a positive leading pulse, not in thephotosensitive direction, for the cases where the cell is illuminated;

FIG. 23 shows the switching response to the second of the switchingwaveforms of FIG. 21, i.e. having a positive leading pulse, not in thephotosensitive direction, for the cases where the cell is unilluminated;

FIG. 24 is a combination of the traces of FIGS. 21 and 22 for the caseof positive leading pulse and on which is identified a region of goodswitching discrimination between illuminated and unilluminatedconditions;

FIG. 25 shows a combination trace for the case of negative leading pulseand on which is identified a region of good switching discriminationbetween illuminated and unilluminated conditions;

FIG. 26 shows a combination trace for the case of negative leading pulseand positive trailing pulse and on which is identified a region of goodswitching discrimination due to leading pulse unilluminated switchingand trailing pulse illuminated switching,

FIG. 27 shows a combination trace for the case of positive leading pulseand negative trailing pulse and on which are identified two regions ofgood switching discrimination, one due to leading pulse unilluminatedswitching and trailing pulse illuminated switching, the other due toleading pulse illuminated switching and trailing pulse unilluminatedswitching; and

FIG. 28 shows three plots of diffraction efficiency for an OASLM devicehaving gratings of fringe pitch 26.8 μm (a), 16.1 μm (b) and 11.1 μm (c)written to the Si side of the device.

The structure of a typical OASLM has been described above with referenceto FIG. 2. Such an OASLM may be used in the Active Tiling holographicdisplay system of FIG. 1, in other types of holographic display systems,or indeed in other systems and applications not related to holography.The following discussion is concerned with the selection of a suitablecontrol signal for application to OASLM to cause the write illuminationpattern to be “written” to the device, so that the pattern cansubsequently be read out from the device by the application of a readlight.

FIG. 5 illustrates a switching waveform of the prior art and itsswitching effect on an FLC OASLM (The cell has a 1.5 μm spaced LC regioncontaining a mixture of 50% SCE8 and 50% SCE8R (racemic) available fromClariant Gmbh). The first pulse is a negative blanking pulse whichswitches all of the LC to one state. The switching pulse is monopolarand its applied polarity is positive, opposite to the photosensitivedirection. Two sets of switching data are plotted, one each for the casewhere the LC is illuminated (light) and un-illuminated (dark), and foreach set the point at which switching begins (speckle) and at which itis complete (clean). Switching takes place above the ‘clean’ line, noswitching takes place below the ‘speckle’ line. The two sets ofswitching curves lie close together and it is not possible to find avoltage and time pulse-width which is within the switching region (above‘clean’ switching line) for one case of illumination and outside theswitching region (below ‘speckle’ switching line) for the other case ofillumination.

FIG. 6 shows the response when the switching pulse of the prior art isapplied in the photosensitive direction. A region is identified andshown shaded in which switching only occurs when the cell isun-illuminated. This device uses a τv_(min) FLC mixture in whichdiscrimination occurs above V_(min), i.e. where the lower voltage causesswitching and the higher voltage does not switch, hence theunilluminated areas of the device switch.

FIGS. 5 and 6 show that the device is working as expected, i.e. thediscrimination occurs when the bias is in the photosensitive direction.

FIG. 7 shows a switching waveform using a (symmetric) bipolar pulse.This is described in the prior art (Applied Optics Vol. 31, No. 32, pp.6859-6868, 10 Nov. 1992) where the OASLM used has ohmic contacts. Whenohmic contacts are used the switching response is the same for eitherpolarity of pulse. The device of our invention does not use ohmiccontacts and this can be seen from the differing responses to pulses ofeach polarity in FIGS. 5 and 6, and 7 and 8.

In FIG. 7 the trailing part of the bipolar pulse, which is the part thatcauses switching, is in the photosensitive direction yet the switchingcurves lie close together and there is no discrimination toillumination. In FIG. 8 the trailing part of the bipolar pulse is not inthe photosensitive direction yet an operating region of discriminationto illumination is identified and is shown shaded.

The effects of varying the shape of the bipolar switching waveform willnow be considered. In particular the effects of making the bipolarwaveform asymmetric will be considered.

FIGS. 9 and 10 show an asymmetric bipolar pulse in which the leadingpart is one tenth the width of the trailing part. FIG. 9 shows that whenthe trailing part of the pulse is in the photosensitive direction thereis no operating window, but if the trailing pulse is not in thephotosensitive direction then an operating window does exist. Thisregion is, again, shown shaded in FIG. 10. The operating window islarger than that identified in FIG. 8 where a symmetrical bipolar pulsewas used. This operating window also occurs at shorter pulse widths(faster switching) than that of FIG. 8.

FIG. 11 shows the switching response of two asymmetric pulses where theleading pulse widths are a half and one third of the trailing pulsewidths respectively. It can be seen that the operating range iscomparable to that of FIG. 8 where a symmetrical bipolar pulse was used.

The underlying mechanism which gives rise to the aforementioned resultswill now be explained. The FLC 7 is shown between the two alignmentlayers 6,8 in FIG. 2. As a consequence of the rubbing applied to the twoalignment layers strong anchoring forces hold the molecules at thesubstrates of the device, but at greater distances from the substratesthe effect diminishes. In the smectic C* phase with C2 alignment thematerial is aligned in a plurality of chevron shaped layers. FIG. 12shows one of these chevron layers. This Figure also shows a so-calledend view of the layer for the sake of completeness. The actualconfiguration occurring between the substrates of the device is verycomplicated. The liquid crystal director lies at a position on thesmectic cone that depends on the distance through the layer. A typicalconfiguration is shown in FIG. 12 for the C2U alignment geometry. In theabsence of any applied field there is a roughly linear variation in thetwist of the director from its position at the bottom (top) of the coneat the upper (lower) surface to a maximum twist at the chevroninterface. This structure is often referred to as the TDP or TwistedDirector Profile.

When a field is applied to the structure a distortion of this directorprofile occurs. The exact profile of the director through the thicknessof the layer then depends on many parameters, including the nature ofthe applied field (a.c. or d.c.), and the electrical, physical andelastic parameters of the liquid crystal material.

There are two stable positions at the chevron interface which arebroadly determined by the value of the smectic cone and the tilt angleof the chevroned smectic layers. Certain orientations of the layerbetween crossed polarisers will give optical differences between the twostates shown in the transmitted light intensity at a given wavelength Alatching operation is said to result when an applied voltage waveformcauses the director at the chevron interface to switch between these twostable positions.

The actual process by which the layer responds to a latching waveform ishighly complicated and may involve a position dependent distortion ofthe director profile, distortion of the chevron layer profile, changesin the smectic cone angle, and the nucleation of 3d domain structureswithin the layer. However, the switching will be here described in termsof a highly simplified model in which the director is at the sameposition described by the angle phi on the smectic cone throughout theentirety of the FLC layer.

The simulation results shown in FIGS. 14 and 15 all make use of thismodel for the switching of the FLC layer and are based on the approachgiven in the paper by P. Maltese and R. Piccolo which appears in theSociety of Information Display conference Digest 1993 on p642.

FIG. 13A shows one of the switching cones around which the molecules (ordirector) of the FLC material can be thought to move. The Figure showsboth of the possible fully-switched positions of the director DC andDC′. FLC devices switch as a result of a net DC field favouring one sideof the cone. The polarisation directors of the molecules, P_(s) andP_(s)′ respectively, are also shown. In practice, however, as will bediscussed below, the director does not occupy these fully-switchedpositions.

FIG. 13B shows a view of the cone from the end thereof (a so called‘plan view’) showing some positions of the director around the conebetween position DC and position DC′. Position DC is denoted at an angleof φ=0° and position DC′ is denoted an angle of φ=180°. Looking at theFigure, the director is assumed to rotate around the cone in a clockwisedirection under the influence of an applied field of a certain polarity.However, the director of the LC molecules will only occupy the positionsDC and DC′ under the continued influence of an applied field of suitablepolarity and sufficient magnitude. When such a field is not present thedirector relaxes around the cone away from the fully switched positionto some extent. In this example the director starts from a relaxedposition at an angle marked φ_(r). Once the director has been switchedto the point φ_(s), exactly halfway between the fully switched positionsDC and DC′, it will continue to move naturally towards DC′ (although itwill eventually come to rest at φ_(r)′) to complete the switchingprocess (at which point the LC is said to be latched). Switching occurswhen the electric field results in a net torque on the director tendingto change φ. The speed of the switching will depend on the magnitude ofthe torque and the total change in orientation through which thedirector moves.

The applied ferroelectric torque is dependent upon the position of thedirector around the cone as shown in FIG. 13C and is also linearlyrelated to the magnitude and direction of the applied field for aparticular director orientation. FIG. 13C shows that for a particularapplied voltage the applied torque reduces as the director moves fromthe position Φ=90° towards Φ=0°.

When the first part of a bipolar pulse, whose trailing part is chosen toswitch the FLC, is applied to the director in its relaxed position,φ_(r), its effect will be to move the director towards the position φ=0.When the trailing part of the pulse is applied a greater torque willthen be required to switch from the new position closer to φ=0. When anillumination pattern of write light is incident upon the device and theleading part of the pulse is in the photosensitive direction then therewill be two new director positions taken up, φ_(I) and φ_(NI), forilluminated and non-illuminated respectively. Since in the illuminatedareas there will be more voltage applied to the LC the director willmove further towards φ=0 in those areas. It turns out that these twodifferent starting positions of the director, which lead to a lowerferroelectric torque being applied in the illuminated region, lead togreater discrimination for switching when the trailing pulse is not inthe photosensitive direction than when the director has a commonstarting position and the trailing pulse is in the photosensitivedirection.

It will be appreciated that if the leading pulse is too large inmagnitude (voltage or pulse width or a combination of the two), it willresult in both the illuminated and unilluminated areas tending towards aposition φ=0 and in this case there will be reduced discrimination andlonger switching times.

Computer modelling of the switching response of an FLC has been carriedout to show the effect of reducing the width or amplitude of the firstpart of a bipolar pulse. As mentioned above the simulation uses asimplified model for the switching of the FLC layer which follows theapproach of Maltese and Piccolo. Results are shown in FIGS. 14 and 15.FIG. 14 shows the response of a bipolar pulse in which the leading partvaries in width with respect to the trailing part. At the two extremes(indicated by broken lines) are the slowest pulse (symmetrical bipolar,leading width=trailing width) and the fastest pulse (monopulse, leadingwidth=zero). Intermediate leading pulse widths give intermediateresponse times and it can be seen that as the leading pulse widthreduces below ˜1/20 of the trailing width, the switching response timeapproaches that of the monopulse. Similar effects are observed whenreducing the voltage of the leading part of the pulse as shown in FIG.15.

The discussion will now turn to the case where the leading part of thepulse and the trailing part of the pulse are both effective forswitching in opposite illumination states, so that the blaking pulse canbe dispensed with. This section will also describe in more detail howmeasurements have been made. In these measurements the switchingcharacteristics of a single pixel device, wholly illuminated or whollyunilluminated, have been measured.

FIG. 16 (top) illustrates schematically two bipolar switching waveforms.For the purpose of obtaining experimental results, the sequence ofwaveforms (i.e. first blanking pulse, first switching waveform, secondblanking pulse, second switching waveform) was applied to the controlelectrode of an OASLM. The advantage of this sequence is that itachieves dc balance across the OALSM device and allows the LC responseto both polarities of bias to be measured. A first of the waveforms 12has a positive leading pulse whilst the second waveform 13 has anegative leading pulse. Each switching pulse is preceded by a blankingpulse 14, 15 of the same polarity as the leading part of the switchingpulse. Thus if the leading part of the pulse causes switching and thetrailing part does not, this will not be detected since the state willbe the same as that switched to by the blanking pulse. This measurementwill only detect switching to the trailing part of the bipolar pulse.The blanking pulses have fixed amplitudes and widths, chosen to ensurefull switching. Immediately following each switching pulse, theintensity provides a measure of the switching resulting from thecorresponding trailing pulse. In FIG. 16, beneath the switchingwaveforms and blanking pulses there is illustrated the correspondingliquid crystal transmission state changes which result from theapplication of the waveforms. The full line following the application ofeach blanking pulse indicates the fully switched states while the dashedlines following each switching pulse indicate the partial switchedstates that exist as the switching voltage is increased from the onsetof switching until full latching occurs.

In order to determine the switching characteristics of an OASLM for theillustrated switching waveforms, switching pulse widths were set and thevoltage amplitude varied so that transmission levels were measured as afunction of voltage at various pulse widths. From this data the voltageand pulse width for 5% and 95% switching was extracted, hence theswitching characteristics in the voltage—time plane were found.Measurements were made with the “silicon side” of the OASLM bothilluminated (by green light) and un-illuminated. The cell was of nominal1.5 μm LC spacing and contained mixture SCE8 available from ClariantGmbH.

FIG. 17 illustrates the results for the condition of no illumination forthe switching pulse having a positive trailing pulse, (i.e. the secondswitching waveform in FIG. 16), not in the photosensitive direction,whilst FIG. 18 illustrates the results for the same waveform under thecondition of illumination.

FIG. 19 is a combination of the traces of FIGS. 17 and 18. The areasshown cross-hatched in the Figure are those areas in which there isswitching discrimination to the trailing pulse under the application ofwrite light. The larger cross-hatched region (to the left of the minimain the switching characteristic) is that region in which the trailingpulse causes 95% or more of the illuminated area to switch (from thestate set by the blanking pulse) but less than 5% of the non-illuminatedarea to switch. The smaller cross-hatched region (to the right of theminima in the switching characteristic) is that in which the trailingpulse causes 95% or more of the unilluminated area to switch (from thestate set by the blanking pulse) but less than 5% of the illuminatedarea to switch. Note that comparisons on the relative size of these tworegions of discrimination should not be made solely on the evidence ofFIG. 19 since there is potentially a larger region of discrimination athigher voltages which has not been measured.

FIG. 20 illustrates the equivalent composite trace for the firstswitching waveform in FIG. 16, i.e. with Si biased negative, thephotosensitive direction, with the cross-hatched areas againillustrating regions of good switching discrimination (for the trailingpulse). As above it is not possible to compare the relative sizes of theswitching windows above and below the minimum in the response time.

An interesting point to note from FIGS. 19 and 20 is that the region ofdiscrimination below the minimum in the response time is larger in FIG.19 than in FIG. 20. It is thus the case that a switching waveform havinga leading pulse in the photosensitive direction and a trailing pulse notin the photosensitive direction is preferable to a switching pulse ofopposite polarity, when operating in the region below the minimum in theswitching characteristic. This is contrary to conventional wisdom whichhas argued that switching with a bipolar waveform will be due to thetrailing pulse, and that the trailing pulse must therefore be in thephotosensitive direction. This result reinforces those given earlierdescribing the first aspect of the invention.

The upper trace in FIG. 21 illustrates an alternative control signalsequence (again achieving dc balance) in which, for each switchingwaveform, the leading pulse has a polarity opposite to that of thepreceding blanking pulse, and the trailing pulse has the same polarityas the preceding blanking pulse. In this case only switching due to theleading pulse alone will be measured, i.e. if there is no switching, orif both parts of the pulse switch, the transmission will be left in thesame state as switched to by the preceding blanking pulse. Thecorresponding change in intensity resulting from this switching sequenceis illustrated in the lower trace in FIG. 21.

The operating regions for the leading pulse are more complex than thosefor the trailing pulse since, for a particular pulse width, trailingpulse switching effectively cuts off leading pulse switching above acertain voltage. This leads to an enclosed region of switching. FIG. 22illustrates the results for the condition of illumination for theswitching pulse having a positive leading pulse, (i.e. the secondswitching waveform in FIG. 21), not the photosensitive direction, andthe area enclosed by the lines is the switching region. FIG. 23illustrates the results for the same waveform under the condition of noillumination.

FIG. 24 illustrates a composite trace for the data of FIGS. 22 and 23.The cross-hatched areas are those in which good switching discriminationcan be achieved for the leading pulse.

FIG. 25 is the equivalent composite trace for the first switchingwaveform of FIG. 21 (i.e. having a negative leading pulse and a positivetrailing pulse).

From these Figures it can be seen that the operating window is largerwhen the leading pulse is in the photosensitive direction. This is whatwould be expected since the leading pulse is effectively behaving as amonopulse, there is no immediately preceding pulse to influence thedirector and move it from its relaxed position. The trailing pulseinfluences the operating window since it will switch back from the stateswitched to by leading pulse under certain conditions of voltage andpulse width.

As already described, the traces of FIGS. 19 and 20 illustrate theswitching characteristics of the trailing pulse of respective bipolarswitching waveforms, whilst FIGS. 24 and 25 illustrate the switchingcharacteristics of the leading pulse of respective bipolar switchingwaveforms. These two sets of characteristics can be used to design abipolar switching waveform which provides switching (of gooddiscrimination) to both leading and trailing parts of the waveform. Inparticular, the characteristics can be used to design a waveform whichprovides switching to one part which reinforces the state imposed by theblanking pulse (in appropriate regions-illuminated or unilluminated) andswitching to the other part to the opposite state (in appropriateregions—illuminated or unilluminated).

Considering firstly a bipolar switching waveform having a negativeleading pulse and a positive trailing pulse (the first switchingwaveform in FIG. 21 and the second waveform in FIG. 16), FIG. 26combines the relevant traces of FIGS. 19 and 25 to illustrate thecombined switching characteristics (to both the trailing and leadingpulses) for this bipolar switching waveform. Similarly, FIG. 27 combinesthe traces of FIGS. 20 and 24 to illustrate the combined switchingcharacteristics (to both the trailing and leading pulses) for a bipolarswitching waveform having a positive leading pulse and a negativetrailing pulse. The cross-hatched regions in FIGS. 26 and 27 illustrateonly those operating regions in which switching occurs to both leadingand trailing pulses with good discrimination; regions of discriminationto one pulse only are not indicated.

Operating within the cross-hatched areas of FIG. 26 (corresponding to aswitching waveform having a negative leading pulse and a positivetrailing pulse), the following switching sequence can be assumed.

For the negative leading part—

-   -   Non-illuminated areas—leading pulse (switching area bounded by        “Δ”)—switch—this reinforces the ‘down’ state already existing        due to the blanking pulse, by moving the director from its        relaxed position to the ‘fully switched’ position.    -   Illuminated areas—leading pulse (switching area bounded by        “o”)—non-switch—this leaves the liquid crystal in the ‘down’        state, but in a relaxed director state since there is no        switching effect.

Positive trailing part—

-   -   Non Illuminated areas—trailing pulse (switching area bounded by        “⋄”)—non-switch—this leaves the director in the ‘down’ state.        Even if the pulse was in the partial switch region, the fact        that the director is in the fully switched state makes switching        more difficult than from the relaxed state. There is no tendency        of the non-illuminated areas to switch to the ‘up’ state,    -   Illuminated—the trailing pulse (switching area bounded by “        ”)—switch—switches the liquid in the illuminated area to the        ‘up’ state.

Note that even if there was no blanking pulse, and whatever the state ofthe liquid crystal before application of the bipolar pulse, it willalways be switched to the desired state since—

-   -   The non-illuminated part is switched ‘down’ during the leading        part of the pulse (if not already in that state)    -   The illuminated part is switched ‘up’ in the trailing part of        the pulse.

A similar explanation can be applied to the operating regionsillustrated in FIG. 27. The operating region within the cross-hatchedarea above 35V corresponds to a switching waveform having a negativetrailing pulse switching where unilluminated and a positive leadingpulse switching where illuminated; the cross-hatched operating regionbelow 25V corresponds to a switching waveform having a negative trailingpulse switching where illuminated and a positive leading pulse switchingwhere unilluminated.

The results disclosed above demonstrate that the use of a bipolar pulseto switch the liquid crystal of an OASLM results in—

-   -   a broader operating range than is achieved when switching with a        monopolar pulse, irrespective of the polarity.    -   Trailing pulse switching bipolar pulse operation results in        greater discrimination when the bias of the trailing pulse is        not in the photosensitive direction.    -   Operation at voltages greater than v_(min) results in a wider        operating region than operating below v_(min).    -   A region of switching to each part of a bipolar pulse which        leads to both illuminated and non-illuminated regions of the        liquid crystal switching to different states. This is highly        desirable as it leads to operation which does not need a        blanking pulse and/or gives improved discrimination to        illumination.

The effectiveness of this technique can be further demonstrated byreference to plots of diffraction efficiency, see FIG. 28. In thisexperiment a diffraction grating was projected onto the Si side of theOASLM and the diffraction efficiency of the image diffracted from the LCside of the OALSM was measured. Three different sets of spacing of thediffraction grating were used. The Figure shows diffraction efficiencyrepresented as vertical bars on a 3-D plot which are plotted on a baseplane of voltage time. Bars which are dotted represent regions in whichonly one part of a bipolar pulse is contributing to switching, i.e.either trailing pulse switching or leading pulse switching. Bars whichare full lines represent regions in which switching is taking place dueto both parts of the bipolar pulse. It can be seen that the efficienciesin the regions where both parts of the bipolar pulse are contributing toswitching are greater than those in the regions where only one part of abipolar pulse was contributing to switching; furthermore as thediffraction grating pitch is reduced the diffraction efficiency ismaintained where both parts of the bipolar pulse are contributing toswitching, whereas where only one part of a bipolar pulse iscontributing to switching the efficiency reduces as the grating pitch isreduced. This is very important for the replay of holographic imageswhere micron scale pixels are required.

It will be appreciated by the person of skill in the art that variousmodifications may be made to the above described embodiments withoutdeparting from the scope of the present invention.

1. A method of controlling the switching of an optically addressablespatial light modulator (OASLM), to a first surface of which a writelight signal is applied and to a second surface of which a read lightsignal is applied, the method comprising: applying a bipolar switchingwaveform to control electrodes of the OASLM during each write cycle suchthat the leading pulse of the waveform applies a voltage across theOASLM which is in the photosensitive direction and the trailing pulseapplies a voltage which is not in the photosensitive direction.
 2. Amethod according to claim 1, wherein the shape and amplitude of thebipolar pulse are such that the trailing pulse causes switching betweenstable states.
 3. A method according to claim 2, wherein the shape andamplitude of the bipolar pulse are such that the leading pulse causessubstantially no switching between stable state.
 4. A method accordingto claims 1, wherein the switching waveform has an asymmetric shape. 5.A method according to claim 4, wherein the duration of the leading pulseis less than that of the trailing pulse and/or the amplitude of theleading pulse is less than that of the trailing pulse.
 6. A method ofcontrolling the switching of an optically addressable spatial lightmodulator (OASLM), to a first surface of which a write light signal isapplied and to a second surface of which a read light signal is applied,the method comprising: applying a bipolar switching waveform to controlelectrodes of the OASLM during each write cycle, one of the pulses ofthe switching waveform causing illuminated areas of the OASLM tosubstantially switch from a first to a second state whilst causingsubstantially no switching of unilluminated areas, and the other pulseof the bipolar waveform causing unilluminated areas of the OASLM tosubstantially switch from the second to the first state whilst causingsubstantially no switching of illuminated areas.
 7. A method accordingto claim 6, wherein the pulse amplitudes and widths are chosen to liewithin that region of pulse amplitude/width space which is substantiallybounded by: a) a line defining between 95% and 100% switching ofilluminated areas to said first pulse of the bipolar waveform; and b) aline defining between 0% and 5% switching of unilluminated areas to saidsecond pulse of the bipolar waveform.
 8. A method according to claim 6,wherein the pulse amplitudes and widths are chosen to lie within thatregion of pulse amplitude/width space which is substantially bounded by:a) a line defining between 95% and 100% switching of unilluminated areasto said first pulse of the bipolar waveform; and b) a line definingbetween 0% and 5% switching of illuminated areas to said second pulse ofthe bipolar waveform.
 9. A method according to claim 6, wherein eachbipolar switching waveform is preceded by a blanking pulse whichswitches the entire OASLM to either said first or second state.
 10. Amethod according to claim 6, wherein the bipolar switching waveform isnot preceded by a blanking pulse.
 11. A method according to claim 6,wherein the switching waveform has an asymmetric shape.
 12. A method ofcontrolling the switching of an optically addressable spatial lightmodulator (OASLM), to a first surface of which a write light signal isapplied and to a second surface of which a read light signal is applied,the method comprising: applying an asymmetric bipolar switching waveformto control electrodes of the OASLM during each write cycle.
 13. A methodaccording to claim 12, wherein the polarity of the leading pulse of thebipolar waveform is in the photosensitive direction.
 14. A methodaccording to claim 12, wherein the pulse width ratio between the leadingpulse and the trailing pulse is at least 1:4.
 15. A method according toclaim 1, wherein the OASLM comprises a liquid crystal having a responsetime for switching between first and second states which depends uponthe voltage across the liquid crystal layer, the response time having aminimum at a given voltage.
 16. A method according to claim 1, whereinthe bipolar pulse switching waveform causes unilluminated areas toswitch and does not switch illuminated areas.
 17. An opticallyaddressable spatial light modulator (OASLM) which in use is arranged tohave a write light signal applied to a first surface thereof and a readlight signal applied to a second surface thereof, the OASLM comprising:a ferroelectric liquid crystal which gives the OASLM device a responsetime for switching between the first and second states which dependsupon the voltage applied across the OASLM and the response time having aminimum value at a given voltage.
 18. A display system comprising anoptically addressable spatial light modulator according to claim
 17. 19.A display system comprising: an optically addressable spatial lightmodulator (OASLM); means for applying a write light signal to a firstsurface of the OASLM; means for applying a read light signal to a secondsurface of the OASLM; and OASLM control means for applying a bipolarswitching waveform to control electrodes of the OASLM during each writecycle such that in use the leading pulse of the waveform applies avoltage across the OASLM which is in the photosensitive direction andthe trailing pulse applies a voltage which is not in the photosensitivedirection. 20-21. (canceled)